Gilbert R. Seely

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN‐C60 DYADS

Abstract Porphyrin‐C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn‐C60) and free base (P‐C60) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C60 The lifetime of Pzn‐1C60 is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+‐C60bull;‐. In toluene, P‐1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P‐1C60 underoes photoinduced electron transfer to give P•+‐C60bull;‐. The electron transfer rate constant is −2 × 1011 s−1.

PHOTOINDUCED ELECTRON TRANSFER IN A CAROTENOBUCKMINSTERFULLERENE DYAD

A carotenoid‐fullerene dyad has been synthesized by condensing a carotenoid amine with an acid group attached to C60 by a cyclopropane‐based linkage. The lowest excited singlet state of the fullerene is strongly quenched by electron transfer from the carotenoid moiety to generate the charge‐separated species Car+‐C60.‐. In CS2 solution Car+‐C60.‐ has a rise time of 0.8 ps and decays by charge recombination in 534 ps. Light absorbed by either chromophore produces a high yield of Car+‐C60.‐, which implies that internal conversion in the carotenoid is negligible. The lowest triplet level in the dyad is localized on the carotenoid and is populated in low yield from the charge‐separated species. The sensitization of singlet oxygen by the fullerene component is effectively curtailed in the dyad.

PHOTOPHYSICAL PROPERTIES OF 2‐NITRO‐5,10,15,20‐TETRA‐p‐TOLYLPORPHYRINS

Abstract— Tetraarylporphyrins substituted with nitro groups at fipyrrolic positions are potential candidates for electron‐accepting pigments in model systems for photosynthesis. The photophysics of 2‐nitro‐5,10,15,20‐tetra‐p‐tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet‐triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature‐ and solvent‐dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+ ‐NO2; in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.

A carotenoid-porphyrin-diquinone tetrad: synthesis, electrochemistry and photoinitiated electron transfer

Abstract A molecular tetrad (C-P-QA-QB) consisting of a carotenoid polyene, a porphyrin and a diquinone moiety has been synthesized. The quinone with the lower reduction potential, the naphthoquinone (QA), was linked directly to the porphyrin and the benzoquinone (QB) was attached in series by a rigid bicyclic bridge. This arrangement was designed to promote a biomimetic sequential electron transfer from the porphyrin to QA, and on to QB. Cyclic voltammetric measurements show two distinct reduction steps at−0.65 and −0.46 V ( vs . SCE) for the diquinone moiety, indicating independence of the benzoquinone and the naphthoquinone components. The initial charge separated state, C-P.+-QA.−-Q-B is formed within 15 ps of excitation and lies about 1.6 eV above the ground state. The final charge separated state C.+-P-QA-QB.− is formed with a quantum yield of 0.23 at room temperature (0.5 at 240 K) and lies ca . 1.1 eV above the ground state. Both parallel and sequential mechanisms for the electron transfer processes are elucidated from studies with model triads which feature a carotenoid, a porphyrin and only one of the components of the diquinone moiety.

FLUORESCENCE QUANTUM YIELDS AND LIFETIMES FOR BACTERIOCHLOROPHYLL c

Abstract— Absorption and emission spectra of bacteriochlorophyll c dissolved in a variety of solvents were measured and fluorescence quantum yields determined from the integrated emission spectra. Values for the emission maxima calculated from the positions and bandwidths of the red absorption band using the Stepanov relationship agreed closely with the experimental values. Fluorescence quantum yields varied between 0.10 in methanol and 0.36 in tetrahydrofuran and in dibutylamine. Fluorescence lifetimes were also determined for bacteriochlorophyll c in four of the solvents, and ranged from 2.7 ns in methanol to 7.6 ns in dimethylsulfoxide.

Photoinduced charge separation in a carotenoid-porphyrin-diquinone tetrad: enhancement of quantum yields via control of electronic coupling

Abstract A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

PHOTOCHEMICAL AND SPECTRAL PROPERTIES OF A PARTICULATE MODEL SYSTEM OF CHLOROPHYLL WITH AMPHIPHILES PREPARED FROM HISTAMINE

Abstract— In the photosynthesis model system described, chlorophyll a at an interface photosensitizes the transfer of hydrogen equivalents from a hydrocarbon phase to an aqueous phase. The hydrocarbon phase, to which chlorophyll is adsorbed, consists of polyethylene particles swollen with tetradecane. The particles are also charged positive by co‐adsorption of dodecylpyridinium iodide. Furthermore, chlorophyll is ligated with the imidazole function of one of several amphiphiles derived from histamine, which may or may not contain a reducible nitroaromatic group that can serve as primary electron acceptor from photoexcited chlorophyll. The fluorescence quantum yield of chlorophyll on these particles is diminished by self‐association of the pigment and by reaction with an oxidizing amphiphile; in the latter case, the quantum yield is correlated with the one‐electron redox potential of the amphiphile. Fluorescence‐lifetime analysis reveals that most excited singlet states of chlorophyll are quenched rather quickly, and that most of the fluorescence comes from a small fraction of chlorophyll with long lifetime. All preparations sensitize the photoreduction of 5,5′‐dithiobis(2‐nitrobenzoate) (DTNB) to the water‐soluble thiolate by hydrazobenzene. When the amphiphile that ligates chlorophyll is not oxidizing, the quantum yield of photoreduction is unrelated to the fluorescence yield of the particles, but may be related to the degree of self‐association of chlorophyll. When the amphiphile that ligates chlorophyll is oxidizing, the kinetics of photoreduction of DTNB require that the electron passes through the primary oxidant to DTNB. Quantum yields for photosensitized reducton of oxidizing amphiphiles in the absence of DTNB have a reversed correlation with redox potential, which can be rationalized in terms of the Marcus theory of electron transfer. All data are most consistently accounted for if the primary photoproduct is an ion pair of chlorophyll and primary oxidant when the latter is available, and a chlorophyll radical ion pair when it is not, both formed by electron transfer from the singlet excited state of chlorophyll.

Reaction of chlorophyll with 2,2′-dithiobis(5-nitropyridine)

Abstract Chlorophyll a reacts photochemically with 2,2′-dithiobis (5-nitropyridine), incorporating 5-nitropyridinethiyl residues. On nmr evidence, substitution occurs principally at the α and δ meso positions, and not at the β. The products are markedly less fluorescent than chlorophyll itself, especially in polar solvents, and it is proposed that fluorescence quenched by electron transfer to the nitropyridine residue.

DODECYLPYRIDINIUM ALKANOATES STABILIZE DISPERSED CHLOROPHYLL

Abstract— In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared‐absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomelic and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen‐bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.

Reversible photochemistry in a chlorophyll model system

A reversible, endothermic photochemical redox reaction, sensitized by chlorophyll a and related compounds, has been demonstrated in a heterogeneous particulate system. The oxidant, 5,5′-dithiobis(2-nitrobenzoate) (DTNB), is photoreduced to the thiolate which remains primarily in an aqueous phase. Reductants are trisubstituted hydrazines, capable of oxidation to tetrazanes in the hydrocarbon particle phase. In the course of three days in the dark, thiolate and tetrazane react to regenerate DTNB in yields approaching 100%. A novelty of the present system is that photoreaction often takes place in discrete rate regimes, which are related to the presence of spectrally identifiable associations of chlorophyll pigments, Mg-containing and free bases. Among the associations that promote photochemical activity are those of chlorophyll and pheophytin with themselves and with each other. Perhaps more active are associations of a Mg rhodochlorin allomerization product of chlorophyll with its free base. Contributing to the associations is the stabilizing presence of amphiphiles that both ligate the Mg of chlorophyll strongly and hydrogen-bond to carbonyls: 2-tridecylimidazole, 2-tridecylimidazoline, and (2-aminoethyl)myristamide. Results of this work demonstrate the possibility of generating reaction center models in an artificial heterogeneous system, and of conducting reversible photochemical reactions with them.